Non-topological 2DEG at the surface of YbB$_{6}$ and Divalent Hexaborides

A recent theoretical prediction of YbB$_{6}$ being an f-d band-inverted mixed-valent topological insulator very similar to SmB$_{6}$ [1] and subsequent angle-resolved photoemission topological interpretations of V-shaped electron pockets [2] are at odds with the previous experimental classification of the material as a divalent small p-d band gap semiconductor. Our angle-resolved photoemission of the (001) surface of YbB$_{6}$ confirms the nearly pure divalency of Yb and demonstrates that in-gap surface electron pockets, with slightly non-parabolic dispersion indicative of a small p-d gap, originate from quantum well states confined to the inversion layer of n-type surface regions with cation termination. Spatial- and time-dependent variations of the surface Fermi-level pinning are shown to be universal polar-surface-driven features of the semiconducting divalent hexaborides including CaB$_{6}$, SrB$_{6}$ and EuB$_{6}$ Also DFT$+$U$+$SO$+$mBJ theoretical band calculations are able to reproduce the experimental energy ordering of a p-d gap existing at E$_{\mathrm{F}}$ above the Yb 4f states with no f-d or p-d band inversions.\\[4pt] [1] H. Weng, et al., PRL 112, 16403 (2014).\\[0pt] [2] M. Xia, et al. arXiv:1404.6217; M. Neupane, et al., arXiv:1404.6814; N. Xu, et al., arXiv:1405.0165.